1. Field of the Invention
The present invention relates to a solid-state image pickup device, a method for driving the solid-state image pickup device, a motion detector, and a camera system and, more particularly to a solid-state image pickup device which successively reads a pixel signal from a pixel section, a method for driving the solid-state image pickup device, a motion detector that uses the solid-state image pickup device as an imaging device, and a camera system.
2. Description of the Related Art
The solid-state image pickup devices are mainly divided into a charge-transfer solid-state image pickup device such as a CCD imaging sensor and an X-Y addressing type solid-state image pickup device such as a CMOS imager. These two types are different in operation in the following point. Specifically, the charge-transfer type solid-state image pickup device concurrently starts storage of signal charge on all pixels and concurrently reads signal charge from all pixels. All pixels have equal storage time (exposure time) of the signal charge. The X-Y addressing type solid-state image pickup device starts storage of signal charge on one row of pixels at a time or one pixel at a time, and successively reads, from the pixels, a signal based on the stored signal charge, by addressing each unit pixel. The storage time of the signal charge is different from pixel to pixel.
Motion detectors for detecting the motion of an object (subject) using an image pickup device as a detection sensor have been developed. When the solid-state image pickup is used as a detection sensor, not only the movement of the object is detected, but also the image of the moving object is recognized. For example, when the motion detector is used to detect the speed of a vehicle running on an expressway, the motion detector not only detects the speed of the vehicle, but also recognizes the type of vehicle and the number and characters printed on a number plate of the vehicle, which may run at a speed above a speed limit.
When the detection sensor is used as a solid-state image pickup device, image data of a prior frame is stored in a frame memory, and the image data of the prior frame is compared with image data of a current frame to detect the motion of the object. Specifically, when the object remains still, the image data in the prior frame and the image data in the current frame coincide with each other. The comparison results become zero. When the object is moving, the comparison results become the ones reflecting the motion.
In the above-mentioned conventional art, the image data in the prior frame and the image data in the current frame are compared with each other. The object moving at a fast speed moves by a long distance within one frame. Although the motion of the object is detected, the image of the object looks doubled, and it is difficult to correctly recognize the image.
The frequency of AC utility power source is different from area to area. For example, the AC utility power source in East Japan is 50 [Hz], while that in West Japan is 60 [Hz]. Illumination provided by a fluorescent lamp operated from the AC utility power source blinks in a sine wave having a frequency twice the frequency of the utility power source. The imaging operation of the image pickup device under the illumination provided by the fluorescent lamp is now considered. In the case of a so-called focal-plane shutter type X-Y addressing solid-state image pickup device which performs exposure on a pixel by pixel basis or on a row by row basis, the storage time is different from pixel to pixel or from row to row. When a high-speed electronic shutter is triggered, a bright streak and a dark streak alternate with each other every row on a screen. This phenomenon is called a flicker.
Now it is considered that an imaging operation is performed at a rate of 30 [frames/s] under the fluorescent illumination driven by the 50 [Hz] AC power source. At any particular pixel, signal reading from each pixel is performed with a period of 33.3 (1/30) [ms], namely, at a timing represented by a ♦ mark in FIG. 1 which illustrates a change in the wave of brightness (intensity) under the fluorescent lamp illumination.
When the high-speed electronic shutter is triggered, the signal of the pixel becomes an output value substantially proportional to the light intensity at the reading operation. There occurs a brightness difference between the bright horizontal streak and the dark horizontal streak, with the bright horizontal streak several times brighter than the dark horizontal streak. The brightness difference appears as a flickering. To reduce the flickering, the shutter speed is set to be n/2A (n=1, 2, 3, 4, . . . ) with the driving frequency of the fluorescent lamp being A, and the storage time of the signal charge at each pixel is set to be n times the flicker period (=1/2A).
For example, with n=1, the shutter speed becomes 1/100 [s], and coincides with the illumination period (1/100 [s]) of the fluorescent lamp which blinks at the sine wave having the frequency twice the power source. At a given pixel, the signal reading is performed from the pixel with the period of 10 [ms] in accordance with the waveform diagram illustrated in FIG. 1. Since the signal intensity during the signal reading becomes equal among pixels, the generation of the flickering is controlled.
For example, if an imaging operation is performed under the illumination of the fluorescent lamp driven at the driving frequency A of 50 [Hz], the shutter speed is set to faster than 1/100 [s]. Since the shutter period is shorter than the period of the fluorescent lamp illumination, the signal reading from the pixel is carried out at a timing different by one peak in the waveform represented by a solid line in FIG. 1. For this reason, the bright horizontal streak is brighter than the dark horizontal streak by an integer multiple. In the 50 [Hz] AC utility power source area, the flicker reduction effect is not obtained when the electronic shutter is triggered at a speed faster than 1/100 [s].
When the imaging operation is performed at a rate of 30 [frames/s] in the 60 [Hz] AC utility power source area, the frequency of the AC utility power source has an integer multiple of the frame rate of the solid-state image pickup device. In principle, the problem of flickering does not occur. However, this is not the case If the frequency of the AC utility power source is not an integer multiple of the frame rate of the solid-state image pickup device in the 60 [Hz] AC utility power source area.